Well if the person is seriously mentally disabled then you could most likely tell from the condition of their body, like VERY short nails, seeming paranoid, or talking to them selves. If you are talking about this and being serious:)
I think crazy/insane people are those who repeat the same behavior expecting different results. Some crazy people are able to disguise their illness, pass it off as an addiction problem, or even worse manipulate you into thinking you are the crazy one. Keep a journal of incidents and consult a professional.
You can tell if someone is crazy when the person manifests in a wild or aggressive way towards others. They always claim they are not crazy and they always get mad when told they are wrong or they are crazy. There is being mentally ill and it is argued when you are mentally ill, you are not crazy. You just need help.